This invention relates to antennas, and more particularly to antennas for radiating a cardioid pattern without the requirement of moving parts, as well as an electronic system for controlling the pattern radiated by the antenna and for providing electrical translation between a reference, such as magnetic north, and the pattern radiated by the antenna.
The present invention is particularly useful for generating or radiating a cardioid pattern which rotates a desired frequency, such as 15Hz for use in a tactical air navigation (Tacan) system. Accordingly, the background of the present invention, as well as preferred embodiments of the present invention, will be discussed with respect to Tacan systems and Tacan principles, although it will be appreciated that the antenna structure an electronic system described and claimed herein may be used in other environments.
Turning first to the fundamentals of Tacan range and bearing systems, standardized and existing ground-located Tacan stations transmit a reply pulse-pair-signal in reply to interrogation from an airborne Tacan set. The time duration of these signals is such that a sufficient and satisfactory reply can be made from a ground station for up to 100 interrogating airborne sources. Each aircraft is provided with a range and azimuth read-out with respect to the ground station location. If only a single interrogation exists at any given time in the overall system, the ground station transmitter continues to transmit random squitter pulses such that a total of about 2,700 pulse pairs still are transmitted. Only a few of the total pulse pairs transmitted are precise time replies to the single interrogation, but the function of the other (squitter) pulses is to fill in the time frame so that there is an effective "carrier" of random time pulses that can be amplitude modulated by the transmitting antenna structure.
As is known, a ground Tacan antenna produces an amplitude modulation of the RF output in a space pattern azimuthly. This pattern is caused to rotate at a basic rate of 15 rotations per second. The structure of the antenna comprises a vertical dipole antenna radiating the RF energy in what would normally be (without other associated structure) an omni, that is, uniform circular pattern of amplitudes. This vertical antenna serves as a shaft around which is mounted two concentric non-conductive cylinders. The two cylinders are physically joined and are simultaneously rotated by a servo-controlled motor at 15Hz. The inner cylinder has imbedded in its wall a single vertical conductor dipole such that an amplitude pattern of the RF radiation is established. The second, or outer, cylinder physically attached to the smaller cylinder, has nine vertical conductors spaced 40.degree. apart imbedded in its construction. The alignment of radiating elements is such that one of the nine elements is radially aligned with the single conductor in the smaller cylinder. Because of selected size and radial locations, each outer conductor creates a small amplitude ripple, the whole of which is fixed in relation to the pattern created by the single inner dipole.
When the above-described assembly is rotated, the RF field amplitudes at all distances on any given radial line are caused to vary in such a manner that when demodulated by a receiver there is a 15Hz major sine wave component and a smaller 135Hz sine wave component that are time-synchronized. At a single point in azimuth, the two wave amplitudes are in-phase because the inner and one outer modulating dipole are radially aligned.
A magnetic or optical pick-up is positioned to generate a pulse at one point each 360.degree. rotation of the antenna assembly. This pulse is called the north burst trigger, and it is positioned such that a specific pulse on-the-air burst is transmitted only when the 15Hz RF amplitude lobe maximum-peak is exactly on centerline with magnetic east radial. Stated another way, for a receiver radially aligned along the north magnetic line from the ground beacon, its demodulated 15Hz sine wave will cross the negative-going inflection zero axis exactly the same time as the north reference burst pulse-code group is in the center of its burst.
Neglecting further reference to the 135Hz wave inasmuch as it is present only to increase the precision of phase (i.e., bearing) measurement accuracy, it will be apparent that relative to the moment or precise time of the north reference burst, every increment of angle around the beacon provides a different, but specific, phase relation. An airborne Tacan receiver, therefore, employs the north reference burst to generate a reference 15Hz sine wave, and then compares this internally generated wave with the received 15Hz demodulated wave in a phase detector to provide a bearing read-out of the magnetic bearing heading to the source beacon.
For further details of Tacan systems, antennas, bearing and range measurements, reference may be made to Electronic Avigation Engineering, by Peter C. Sandretto, published by ITT Corporation, 1958. In addition, U.S. Pat. No. 3,474,449 entitled "Phase Angle Measurement System" describes a system for computing or determining the bearing of a movable vehicle from a beacon, such as a Tacan beacon, and illustrates in FIG. 4A thereof the typical Tacan ground beacon radiation pattern. Additionally, U.S. Pat. Nos. 3,281,843, 3,474,447, 3,560,978, 3,670,336, 3,747,102, 3,790,943, 3,795,914, 3,797,019 and 3,863,255 describe antenna systems of interest.
Turning now to a discussion of air-to-air bearing measurements, when a Tacan set is to receive a signal to provide a magnetic bearing from another airborne Tacan signal source, it is evident that the source must be able to provide a 15Hz rotating pattern of amplitude modulation in much the same form as supplied by a ground Tacan beacon station. Since the 15Hz pattern rotation is created by the antenna structure and not by the Tacan electronic system, it follows that the airborne antenna must generate the needed 15Hz rotating pattern to complete the airborne Tacan system package. In addition, some means must be provided to relate or translate the 15Hz pattern to the aircraft heading in order to properly initiate the north burst trigger input to the airborne Tacan transmitter.
Several approaches have been proposed in the past for providing an airborne antenna for radiating or generating the 15Hz pattern. One approach is the provision of a mechanically rotated antenna structure, similar in concept to the conventional ground beacon, installed on the aircraft for generating the appropriate rotating field pattern. However, size, weight, power demand and reliability are drawbacks. In addition, another is an electrically rotated version employed 12 or more pattern-producing digitally switched elements. An example is described in an article entitled, "New Tacan Antennas Offer Gains" by Kenneth J. Stein which appeared at pages 34-37 of the July 24, 1972 issue of Aviation Week & Space Technology. Particular drawbacks of a device of this nature are the physical and electronic structure which is not sufficiently broadband for many Tacan applications, the stepped wavefrom produced which is limited by the number of switched elements, and the higher energy losses induced by the greater number of parasitic elements even though they are in a switched-off condition.